FIG. 1 is a schematic diagram of one form of radio receiver. The receiver comprises an antenna 1 connected to an amplifier 2 which amplifies the radio frequency signal received by the antenna. The output of the amplifier is filtered by a bandpass filter 3. The filtered signal, still at radio frequency, is then downconverted by mixing in mixer 4 with a signal generated by oscillator 5. In the receiver of FIG. 1 the mixer 4 downconverts the radio frequency signal down to an intermediate frequency (IF). The intermediate frequency signal is then downconverted to another intermediate frequency or to baseband by mixing in mixer 6 with a signal generated by oscillator 7. Bandpass filters 8 and 9 are used between mixer 4 and mixer 6, and after mixer 6. The IF or baseband signal is passed to demodulator 10 for demodulation to determine the symbol stream encoded in the signal.
One form of modulation is frequency shift keying (FSK). In FSK modulation symbols are encoded into the signal as variations (shifts) in the signal's frequency about a centre frequency. Especially when the signal is modulated using FSK modulation it is important that the received signal as input to the symbol estimator port of the demodulator is not displaced in its centre frequency with respect to the nominal value of the carrier frequency. Otherwise, the possibility of signals being decoded incorrectly is increased. Such a displacement may arise, for example, from errors in the transmission frequency or in the mixing frequencies used in the receiver (e.g. as generated by oscillators 5 and 7).
In order to cope with displacement of the centre frequency of the received signal, the demodulator may estimate the displacement and apply compensation to the signal or adjust the, demodulation process with the aim of avoiding demodulation errors due to the displacement.